Optical Appearance of Scalarized Kerr-Newman Black Holes with Multiple Light Rings

TL;DR

This paper explores the unique optical signatures of scalarized Kerr-Newman black holes, revealing additional light rings and complex image structures that differ from standard Kerr black holes, potentially aiding future observational identification.

Contribution

It introduces a detailed analysis of light rings and optical images of scalarized Kerr-Newman black holes, highlighting new features like inner photon shells and crescent-shaped images.

Findings

Scalarized Kerr-Newman black holes can have multiple light rings.

Inner photon shells create additional critical curves and higher-order images.

Distinct crescent-shaped images differ from Kerr black hole images.

Abstract

This study investigates the optical appearance of rotating scalarized Kerr-Newman black holes in the Einstein-Maxwell-scalar theory with exponential coupling. By analyzing equatorial null geodesics, these black holes are classified into six types according to the number and properties of their light rings. Combining slow-rotation analysis with full numerical ray tracing, we investigate images of black holes illuminated by a geometrically and optically thin accretion disk. Unlike the Kerr case, where the image is typically governed by a single outer photon shell and a single critical curve, scalarized Kerr-Newman black holes can develop an additional inner photon shell outside the event horizon. This extra shell gives rise to an inner critical curve inside the usual outer one, which may be absent, partial, or closed depending on the black hole parameters and the observer's inclination. Moreover, it generates new higher-order images in the region between the two critical curves, some of which exhibit crescent-like morphologies distinct from the nearly circular higher-order images familiar from Kerr black holes. These features enrich the optical appearance of scalarized black holes and could serve as distinctive observational signatures in future high-resolution observations.